Sang-Joon Ahn

Research Assistant Professor

Main Interests

My research area is to elucidate the regulatory pathways of oral bacteria, with particular focus on biofilm formation and stress tolerance that can modulate the virulence of oral bacteria and enhance the pathogenic potential of oral biofilms. As a standpoint protein for this research, I have been working on AtlA protein, an apparent autolysin of Streptococcus mutans, which was originally identified as a surface-associated protein in our laboratory. AtlA protein is absolutely required for normal biofilm formation, biogenesis of a normal cell surface, and autolysis in S. mutans. AtlA activity is subject to control by the VicRK two-component system (TCS), which is known to play critical roles in pneumococcal virulence. Continuous studies have addressed several key issues regarding the regulation of AtlA and provided further support for the interconnectedness of this pathway with the expression of other virulence attributes and pathways, including genetic competence and exopolysaccharide production by S. mutans. More recently, oxygen was revealed to be a key environmental factor that strongly influences cell envelope composition and biofilm development via the VicRK TCS and the AtlA pathway. Studies are ongoing to further dissect the regulation and processing of AtlA and to evaluate the potential utility of AtlA as a target to disrupt S. mutans pathogenesis.

Stemming from the original AtlA work, another major research area is to investigate the molecular basis for regulation of autolysis of S. mutans by gene products that have major effects on traits related to virulence. This research focuses on the characterization of a pair of dicistronic operons, lrg and cid-like, that significantly affected autolysis and biofilm formation by S. mutans. The pair of operons shares structural features with the bacteriophage-encoded holin family of proteins that are involved in the control of bacteriophage-mediated cell death and lysis. Thus, research on this system has led to the proposal that it comprises the molecular components of bacterial programmed cell death (PCD), which is likely to interconnect with the ability of this organism to rapidly and efficiently cope with changes in the environment. The development of biofilms is believed to involve the programmed elimination of cells by bacterial PCD, or autolysis. Indeed, the ability to eliminate individual bacterial cells in response to damage induced by a variety of harmful or stressful conditions may be critical for survival and persistence of pathogenic biofilms, including S. mutans. In addition to controlling autolysis, the lrg/cid-like operons play unique and important roles in S. mutans, and are regulated in a highly complex manner with multiple different modes of control, including a signal transduction system, quorum-sensing, and carbohydrates. Better understanding of how the Lrg/Cid-like system of S. mutans may contribute to establishment, persistence and virulence in oral environments, will provide a comprehensive picture of the way in which S. mutans integrates environmental or physiological signals during biofilm maturation, and ultimately enhance our understanding of the pathogenic properties of S. mutans and illuminate the way to new strategies to control oral diseases.